Electric drive units are used in many application areas for implementing activation processes of moving parts which are carried out by means of external force. For example, various movable parts (adjustment parts, actuating elements) are activated in seats in vehicles by means of electric drive units, (e.g., seat length adjustment, seat backrest adjustment, seat depth adjustment, seat height adjustment, etc.). The electric drive units have an electric motor (for example, a DC motor) for generating and making available electric drive power, a gear mechanism for transmitting the movement of the electric motor, a device for mechanically coupling or adapting the electric motor or gear mechanism to the movable part (adjustment part, actuating element), in particular for converting the rotational movement of the electric motor into a linear movement, and an electronic motor for actuating and monitoring the electric motor (for example for performing closed-loop control of the rotational speed and power of the electric motor).
The drive force that is required for the electric drive unit is generally between 100 N and 1000 N. In order to increase the comfort for the user or operator, the activation processes which are carried out by means of external force (by electric motor) can often be carried out automatically: the user or operator then simply has to initiate the deactivation process and can dedicate himself to other activities during the (fully) automatically occurring activation process. However, in the case of activation processes which occur automatically, in particular in the case of automatically occurring activation processes which are not monitored further by the user or operator or are triggered by the user or operator from a relatively large distance (by remote control), there is the risk of body parts or objects becoming trapped. Such automatic adjustment processes can, for example, be initiated when a key for calling a stored position is activated so that the electric drive units automatically move the adjustment parts which are to be adjusted (backrest, seat cushion, head rest, etc.) of the vehicle seat into the previously stored position.
Document DE 196 38 781 A1 presents an actuating drive with anti-trapping protection. In order to implement an anti-trapping protection, direct methods or indirect methods can be used: in direct methods of the activation travel which is carried out by the movable part is self-monitored for potential obstacles (for example by providing switching strips or by optical means), which, however, entails high costs and is susceptible to faults; in indirect methods, the trapping force which is produced when trapping occurs is monitored in that trapping is assumed to be occurring if a predefined limiting value (triggering threshold value) for the drive force (starting from a certain excess force) is exceeded; after trapping has been detected, specific measures are initiated, in particular the electric drive unit (the electric motor) can be reversed or switched off.
The trapping force or excess force which is evaluated in the indirect methods for providing anti-trapping protection can either be determined directly (which is extremely costly owing to the sensors, for example force sensors or torque sensors, which are required for this purpose), or indirectly by sensing and evaluating the measured values of at least one engine characteristic variable that is characteristic of the loading of the electric motor or of the torque that is currently output by the electric motor (for example by evaluating the drive speed and/or current drain and/or power drain and/or energy consumption of the electric motor), i.e., by evaluating the recovery of the trapping force to the electric motor of the electric drive unit (change of the drive load).
In this indirect determination of the trapping force, DE 44 42 1 71 A1 discloses that in order to monitor a system with an electromotive drive the frictional force of the drive is taken into account. The frictional force is detected at a time at which trapping does not yet occur, and therefore no trapping force occurs, in which case the frictional force can be determined either indirectly from the motor characteristic variables of the motor current (armature current) and/or motor voltage and/or motor speed or directly by means of suitable sensors on the electric motor.
EP 1 299 782 B1 discloses a method for providing anti-trapping protection in which, in order to monitor the electric drive unit for potential trapping, friction force profiles are determined as a profile of the frictional force during the activation process (either by direct measurement of the forces or by indirect determination by reference to the motor characteristic variables of motor current and/or motor voltage and/or motor speed or period length). These frictional force profiles are placed in a relationship with one another by various successively occurring activation processes in the form of a quality measure; the proportion of the frictional force, and therefore of the trapping force, is sensed in such a way that the triggering threshold value is set to relatively low values as a function of this quality measure. The triggering threshold value can (depending on the quality measure) either be influenced here in a global fashion (independently of position, i.e., independently of the activation travel) or be adapted in a position-dependent fashion by taking into account the activation travel.
WO 02/15359 A1 discloses a method for performing open-loop and closed-loop control of an adjustment device which is driven by motor, in particular a seat adjustment means for a motor vehicle with anti-trapping protection. If a predefined load limit is exceeded, the motor drive is switched off or is adjusted by closed-loop control to a value below the load limit.
Input variables or state variables of the motor drive are acquired continuously. The current loading of the adjustment device or of the motor drive is determined from the state variables by means of a mathematical model of the drive.
By acquiring the relevant state variables of the adjustment device, the relationships between the respective components of the adjustment system are taken into account using the mathematical model of the adjustment device, permitting the trapping force to be calculated very precisely. The adjustment device is to be understood here as referring to all components of an adjustment system which are necessary to operate an adjustment device.
The state variables of the adjustment system are obtained, inter alia, from the changing environmental conditions such as temperature, moisture or pressure, a change in which entails a corresponding change in the adjustment force to be applied for the operation of the adjustment apparatus. The trapping force which can be tolerated at the respective time is determined by means of the adjustment force to be applied so that it is possible to react very precisely if a case of trapping occurs and the drive is to be stopped or reversed.
In this context, it is also possible by calculating the trapping force, by means of the mathematical model of the adjustment apparatus, to take into account influencing variables which cannot be acquired by sensors. It is therefore possible to take into account all the influencing variables or state variables of the system which are acquired in the model and, if appropriate, evaluate the effect on the entire system in terms of intensity.